यह आदत बदलना मुश्किल है।

Hindi word order is usually Subject/Object + Verb, and the verb often comes at the end.

So this sentence is built like:

• यह आदत = this habit
• बदलना = to change / changing
• मुश्किल = difficult
• है = is

So the overall structure is:

• यह आदत बदलना मुश्किल है
• literally: This habit changing difficult is
• natural English: Changing this habit is difficult

Hindi often puts the action phrase before मुश्किल है.

यह means this.

In this sentence, it is being used as a determiner before आदत:

• यह आदत = this habit

A useful comparison:

• यह = this
• वह = that

So:

• यह आदत = this habit
• वह आदत = that habit

Hindi does not use articles like a, an, and the the way English does.

So आदत can mean:

• habit
• a habit
• the habit

The exact meaning depends on context. Here, because of यह, it specifically means this habit.

The -ना form is the infinitive form of the verb, similar to to change in English.

• बदलना = to change

In this sentence, the infinitive is being used like a verbal noun or action phrase:

• यह आदत बदलना = changing this habit / to change this habit

Hindi often uses the infinitive this way when talking about an action in a general sense:

• तैरना अच्छा है = Swimming is good
• जल्दी उठना मुश्किल है = Waking up early is difficult

So बदलना is correct because the sentence is talking about the act of changing the habit.

Because बदलना here is not agreeing with आदत as an adjective. It is the infinitive verb to change.

So:

• आदत is feminine
• but बदलना stays in the infinitive form

It is not describing the habit; it is naming the action.

Compare:

• अच्छी आदत = a good habit
  
  • here अच्छी agrees with आदत
• आदत बदलना = to change a habit / changing a habit
  
  • here बदलना is a verb form, so no feminine agreement happens

मुश्किल is an adjective/noun borrowed from Persian/Arabic, and in modern Hindi it usually stays the same form.

So you say:

• यह काम मुश्किल है = This work is difficult
• यह आदत मुश्किल है would sound odd in meaning, but मुश्किल itself does not change form
• यह किताब मुश्किल है = This book is difficult

In your sentence, मुश्किल is describing the action phrase यह आदत बदलना as difficult.

You can also use a more Sanskrit-based adjective like कठिन:

• यह आदत बदलना कठिन है = Changing this habit is difficult

Both are correct, though मुश्किल is very common in everyday speech.

Yes, इस आदत को बदलना मुश्किल है is also very common, and many learners may actually find it clearer.

Here is the difference:

• यह आदत बदलना मुश्किल है
• इस आदत को बदलना मुश्किल है

Both mean essentially the same thing: Changing this habit is difficult.

Why the change?

• यह आदत = this habit
• but when a postposition like को is added, यह changes to its oblique form:
  • यह → इस
• so:
  • इस आदत को = this habit (as an object)

The version with को makes the object relationship more explicit.
The version without को is more compact and still natural in many contexts.

Because Hindi pronouns and demonstratives often change form before postpositions.

This is called the oblique form.

Examples:

• यह = this
• इसका = of this
• इसको / इसे = to this / this
• इस पर = on this

So:

• यह आदत = this habit
• इस आदत को = this habit (with को)

This is a very common pattern in Hindi.

The subject is not just यह आदत by itself. The whole action phrase functions as the subject:

• यह आदत बदलना = changing this habit

That whole phrase is what is मुश्किल है.

So the structure is:

• [यह आदत बदलना] = subject-like action phrase
• मुश्किल है = is difficult

This is similar to English:

• Changing this habit is difficult.

Because the whole action phrase यह आदत बदलना is being treated as a single idea.

Hindi often uses है with infinitive phrases like this:

• दौड़ना अच्छा है = Running is good
• सच बोलना ज़रूरी है = Telling the truth is necessary
• यह आदत बदलना मुश्किल है = Changing this habit is difficult

Even though आदत is a noun, the sentence is really about the act of changing it, which is treated as a single unit.

Usually बदलना means to change or to change/be changed, depending on context. Hindi often allows the same root to be used in slightly different ways.

In your sentence:

• यह आदत बदलना मुश्किल है

the meaning is clearly:

• to change this habit
• not for this habit to change by itself

If you wanted to be more explicitly transitive, Hindi often still uses बदलना in everyday speech. In some contexts, learners may also see related forms like:

• बदल देना = to change completely / to alter
• बदल जाना = to get changed / to change on its own

But here, plain बदलना is normal.

It is understandable and natural enough, but many native speakers would probably more often say:

• इस आदत को बदलना मुश्किल है।
• यह आदत बदलना बहुत मुश्किल है।
• इस आदत को बदलना आसान नहीं है।

The original sentence is fine, but adding को often sounds a bit more explicit and natural to learners’ ears.

So these are all good versions:

• यह आदत बदलना मुश्किल है।
• इस आदत को बदलना मुश्किल है।
• यह आदत बदलना बहुत मुश्किल है।

A simple pronunciation guide is:

• यह = yah
• आदत = aa-dat
• बदलना = ba-dal-naa
• मुश्किल = mush-kil
• है = hai

So roughly:

yah aa-dat ba-dal-naa mush-kil hai

A more natural connected rhythm would be something like:

yah aadat badalna mushkil hai

In normal speech, यह is often pronounced closer to ये / yeh in many accents, though the spelling remains यह.